The present invention pertains to the field of impedance control. More particularly, the present invention pertains to adjustment of a circuit impedance relative to a transmission line impedance.
The impedance of a driver and receiver relative to the impedance of a transmission line has a significant impact on the efficiency and reliability of communications across the line. For example, integrated circuit chips (IC""s) are increasingly operating at such high frequencies that the signal leads and interconnecting traces between chips behave like transmission lines rather than lumped capacitors. The transmission lines have a characteristic impedance (Zo) which needs to be considered for efficient and reliable transmission across the line. A driver transmitting signals through a transmission line to a receiver or multiple receivers can be plagued by a variety of problems.
For example, multiple reflections due to impedance mismatches at the driver-transmission line interface and the receiver-transmission line interface(s) can cause errors in decoding the signal received at the receiver. These errors can be due to signal integrity factors, such as monotonicity, overshoot, ringback, and long settling times which may impact the signal flight time.
While impedance mismatch creates problems, matching the impedance at the driver-transmission line interface has its own drawback, as well. For example, matching the driver impedance to the transmission line impedance may result in the signal having a large rise time due to the relatively high resistance-capacitance (RC) time constant formed by the driver impedance and the driver output capacitance. Large rise times are undesirable for several reasons, including the increased susceptibility to noise during the time needed for the signal to reach the threshold region of the receiver, and increased driver switching times.
Adjusting the impedance at the driver or the receiver relative to the line impedance is important for controlling the performance across a transmission line. The performance of an IC chip is significantly affected by the impedance of the driver and the receiver. The switching times, number and strength of reflections, and switching quality are a function of the impedance of the driver and the receiver relative to the transmission line impedance. Since decreased switching times, fewer and weaker reflections, and acceptable signal quality would improve performance, it would be advantageous to control impedance so as to achieve relatively lower switching times while still achieving acceptable signal quality.
According to an embodiment of the invention a method of adjusting a circuit operating characteristic is described. For one embodiment, the circuit operating characteristic can be an impedance. The method includes generating a first signal for application to a reference termination. The method then includes generating a first voltage based on the first signal at a first point on the reference termination and generating a second voltage based on the first signal at a second point on the reference termination. The method also includes adjusting an operating characteristic based upon the first voltage and the second voltage.